Muscle Structure

MUSCLE: STRUCTURE TYPES • On structure • Striated: have large number of cross striations • Non striated: no cross striations • On situation • Skeletal • Cardiac • smooth COMPOSITION OF DIFFERENT MUSCLES SKELETAL MUSCLE • Striated, voluntary muscles • Surrounded by fascia • Covered by connective tissue called epimysium • Arranged in bundles called fasciculi covered by perimysium • Each fibre is covered by connective tissue sheath called endomysium SKELETAL MUSCLE • Made up of organization of cells also called fibers which are known as myocytes • Length is between 1cm – 4cm • Enclosed in a cell membrane called sarcolemma and contains sarcoplasm • Sarcoplasm contains cellular organelles • Each fiber may be multinucleated • Contains filaments called myofibrils • Attached to tendons which are attached to bone MYOFIBRIL • Parallel filaments that run through the entire length of the fiber • Located in the sarcoplasm • Length depends on the length of the muscle fiber • Consists of alternating bands/segments/discs formed by muscle proteins • Light band or I band • Dark band or A band • Light band isotropic to polarized light • Dark band anisotropic to polarized light • Bands give the appearance of cross striations • I band contains a dark portion called a z disc, this does not allow the passage of light • The area between Z discs is sarcomere and is the contractile unit of muscle SARCOMERE • Structural and functional unit of skeletal muscle • Each myofibril contains many sarcomeres arranged end to end throughout it’s length • Within the A band of the myofibril is an H zone containing the middle part of a myosin filament. • Sarcomere consists of myofilaments • Actin • myosin ACTIN • Actin filaments are the thin filaments with a diameter of 20 Å and a length of 1 μ. • The bases are inserted strongly into the Z discs • extend from either side of the ‘Z’ lines, lying in the spaces between the myosin molecules, • Composed of double stranded F actin molecule • The two strands are wound in a helix • Each F actin molecule is made up of polymerized G actin molecules. Each with a molecule of ADP attached. These form the active sites where it forms crossbridges with myosin filaments • Also contain a tropomyosin molecule which wraps around the F actin molecule In a spiral fashion • In resting phase, it covers the active site of the molecule, preventing attraction to myosin • Tropomyosin is attached to the F actin molecule by troponin molecule (troponin I, troponin C and troponin T). It has a strong affinity for calcium ions MYOSIN • thick filaments with a diameter of 115 Å and a length of 1.5 μ • Situated in ‘A’ band. • Composed of multiple myosin molecules • Each composed of 6 polypeptide chains 2 heavy and four light • The chains are wound in a helical structure composed of a folded part called the head and the wound part called the tail • The filament is made up of 200 or more molecules • The filament is arranged with the myosin heads in axial displacement of 120 degrees from the preceding head. • Myosin heads also act as an ATPase enzyme. • Actin and myosin filaments form crossbridges which shorten during muscle contraction. • There are other proteins contained in the sarcomere • Actinin which attaches actin filament to Z line • Desmin, which binds ‘Z’ line with sarcolemma. • Nebulin, which runs in close association with and parallel to actin filaments. • Titin, a large protein connecting ‘M’ line and ‘Z’ line. Each titin molecule forms scaffolding (framework) for sarcomere and provides elasticity to the muscle. SARCOTUBULAR SYSTEM • A system of membranous vesicles and tubules surrounding the myofibrils in the sarcoplasm. Composed of T tubules and L tubules • T tubules are narrow tubules which penetrate through the muscle cell and open to the exterior, thus ECF runs through their lumen • T tubules transfer action potential to the myofibrils. • Skeletal muscle also contains myoglobin within the sarcolemma which serves to transport oxygen • L tubules also called the sarcoplasmic reticulum are a closed tubular system around each myofibril and does not communicate with cell exterior • L tubules dilate to form a sac called terminal cisternae • L tubules store calcium SMOOTH MUSCLE • Non striated, involuntary muscles • Found in organs • Small, fusiform or elongated cells 50 – 200 microns in length • Single, centrally placed, elongated nuclei with 2 or more nucleoli STRUCTURE • Contractile proteins actin, myosin, and tropomyosin • Thick filaments formed by myosin molecules • Thin filaments formed by actin and tropomyosin • Filaments are scattered in the sarcoplasm • They contain more cross bridges than skeletal muscle • Dense bodies are scattered in the cytoplasm and are special structures to which actin filaments attach. • Filaments have a sidepolar arrangement and thus can contract in a corkscrew fashion. • Well defined myofibrils and sarcomeres are absent • T tubules are absent and L tubules are poorly developed TYPES OF SMOOTH MUSCLE FIBRES • Single unit fibers/unitary smooth musc le/syncytial smooth muscle are a mass (hundreds) of smooth muscle that contact together as a single unit • Cells are arranged in bundles or sheets and are adherent to one another at multiple points • Cell membranes are joined by gap junctions which allow free flow of ions from one cell to the next • Multi unit smooth muscle • Made up of discrete, separate, smooth muscle fibers and are often innervated by a single nerve ending • No gap jucntions between cells • Covered by a thin layer of basement membrane that helps separate cells from one another CARDIAC MUSCLE • Similar to skeletal muscle • Cells are arranged in series and in parallel • Cells are connected by intercalated discs which have gap juctions that allow rapid diffusion of ions • Thus cardiac muscle is also a functional syncytium.